Japanese patent application publication No. 11-248540A discloses a conventional infrared ray detector. The conventional infrared ray detector comprises a housing, an infrared ray detection element, and a condenser lens. The housing houses the infrared ray detection element. The housing is formed with an opening such that the condenser lens is attached to the housing to be located in the opening. The infrared ray detection element is arranged to receive infrared ray. The infrared ray detection element is configured to output the electrical signal according to an amount of the infrared ray incident on the infrared ray detection element. The condenser lens is disposed to concentrate the infrared ray incident on the condenser lens from a predetermined detection area into the infrared ray detection element. The infrared ray is incorporated into lighting devices, automatic doors, and security devices in order to detect the infrared ray radiated from the human. The infrared ray detector is configured to generate the signal on the basis of the detected infrared ray, and subsequently send the signal to the control unit. The control unit receives the signal, and is configured to control the lighting devices, the automatic doors and the security devices on the basis of the signal. In a case where the infrared ray detector is employed in order to control the devices such as the lighting devices, the infrared ray detector is configured to monitor a plurality of detection areas. Consequently, the infrared ray detector is configured to detect the human being located in the detection areas.
The infrared ray detector requires sets of the infrared ray detection element and the condenser lens per detection area in order to detect existence or nonexistence of the human in a plurality of the detection areas. Consequently, the infrared ray detector is configured to have the number of the detection areas which is equal to the number of the condenser lenses. In the infrared ray detector, the infrared ray radiated from the human in the detection area corresponding to the condenser lens is concentrated into the infrared ray detection element. The infrared ray detection element detects the concentrated infrared ray. In this manner, the infrared ray detector detects the infrared ray. However, the infrared ray detector of this type requires a plurality of the condenser lenses. That is, the infrared ray detector of this type requires a lot of components. Therefore, there is a problem that cost of the infrared ray detector is increased.
Moreover, Japanese patent application publication No. 10-214546A discloses another conventional infrared ray detector having a diffraction grating. In this infrared ray detector, the infrared ray from the detection area is passed through the diffraction grating to the infrared ray receiving unit. However, the diffraction grating has a narrow viewing field for detection area. Therefore, there is a low flexibility of designing the detection area.